This invention relates to trimming blow molded articles and more particularly to trimming flash from such articles with a rotating blade as they traverse a cutting path.
Removing flash formed integral with the usable portion of an unfinished blow molded article downstream of the molding station by means of a knife penetrating the plastic at the desired level as the article advances along a path between fixed and moving guide members is known in the art. For example, a system employing a linear path is disclosed in U.S. Pat. No. 3,429,211 whereas systems utilizing a curved path are disclosed in U.S. Pat. Nos. 3,406,598; 3,675,521 and 3,800,638. Though both are well used in the industry, primarily because of simplicity of equipment and process design, the type using a rotating wheel as the moving guide member has the advantage amongst others of providing a truly solid support for the article on the moving side of the cutting path. This minimizes slippage of the part on the moving surface during sectioning and the concomitant problem of variability in spacing of the parts which could result in contact and sticking together of adjacent pieces if the plastic is at a somewhat elevated temperature and not quite set. Also, when support during trimming is in the flash area and it is intended that the trimming part fall by gravity into a waiting holder moving in phase beneath it as it traverses the cutting path, as in the case of the system described in commonly owned copending U.S. application Ser. No. 467,378, filed May 6, 1974, such slippage can result in misalignment of the part with such holder so that when it does fall in an off-center manner, it randomly strikes some surface of the receiving means not really meant to act as the support member and undesirably topples to the side out of the system.
Such copending U.S. Ser. No. 467,378, filed may 6, 1974, discloses a fixed blade trimming assembly utilizing a rotary drive wheel particularly suitable for sectioning parts at elevated temperature where the material is pliant and not fully set. The temperature range of the plastic in the region of the cut as therein specified should be controlled to give optimum cutting - i.e. if the plastic is too cold cracking can occur, whereas if too hot, stretching across the blade surface occurs resulting in an incomplete cut. Though such assembly functions well it has a few deficiencies. More specifically, the elevated temperature range within which an acceptable quality cut is attainable is somewhat narrow and difficult to consistently maintain, especially when upstream blow mold cooling conditions which set the temperature of the part to start with, vary somewhat. It would therefore be desirable from the standpoint of process flexibility to broaden such operating range without sacrificing the quality of the cut.
U.S. Pat. No. 3,406,598 discloses a trimming system of the general type under consideration which cuts on an arc using a wheel as the moving guide with the blade integral witht the wheel. Such system would appear adequate when the finished articles do not require particularly close tolerances on the top surface, as, for example, when the trimmed article is to remain open in use, or if a container, is not intended to hold any substantial pressure. However, when the end use requires a close, cooperating fit of the trimmed surface with another member, or in other words, when extremely close tolerances must be held on such surface, e.g. when meant to cooperate later with a flush, close-fitting closure adequate to contain substantial pressure within the finished article, the system disclosed in such patent is deficient. This is because the vertical location of the cut in the referenced system is determined by the relative positions of the knife and a molded groove in the article, and the surface of such groove can vary slightly depending on the degree of shrinkage which has occurred in the material as determined by its temperature, which shrinkage can vary in successive articles at the time of arrival at the cutting station. Such vertical variability is especially serious when the article is purposely at a high temperature yet within a variable range, and sag due to weakness of the dimensionally unstable plastic is pronounced and variable. Also, since the knife is necessarily larger than the supporting wheel in the referenced system so that cutting can occur, penetration commences a fraction of a second before the article is supported on the blade side, and consequently sidewise canting of the part occurs which results in the trimmed surface deviating somewhat from a truly horizontally flat, planar objective. Also, the blade being integral with and larger than the wheel, penetration through the full wall commences immediately on contact and this produces a sort of splitting or wedging type of penetration resulting in a rough, wavy, trimmed surface not especially suitable for flush, face-to-face contact with a subsequently applied closure.